Implement Bazel path mapping for cross-mode cache sharing

[rejuvenile]

Summary

• Adds supports-path-mapping execution requirement to Rustc and CargoBuildScriptRun actions
• Uses File objects in Args instead of path strings so Bazel's path mapper can rewrite output directory prefixes
• Adds --set-env flag to process wrapper for path-mapped environment variables
• Updates get_linker_and_args() to return the linker File object for path mapping
• Updates rust_std_paths to use File depset with map_each callback

Motivation

Bazel's --experimental_output_paths=strip enables cross-configuration cache sharing by mapping output paths (e.g. bazel-out/k8-fastbuild/bin/... and bazel-out/k8-opt/bin/... both map to bazel-out/cfg/bin/...). This requires actions to:

1. Declare supports-path-mapping in execution requirements
2. Pass all file paths through Args objects (not literal strings) so Bazel can rewrite them

rules_rust was using .path strings in many places, preventing path mapping from working. This PR converts the key paths to use File objects in Args:

• --out-dir uses map_each = _file_dirname callback
• OUT_DIR is passed via --set-env (a new process wrapper flag) instead of the env dict
• --codegen=linker= uses the File object when available
• -L (stdlib paths) uses map_each = _file_dirname with uniquify = True
• --output-file uses File objects

Build script environment variables containing file paths are passed through --extra_env command-line args (which Bazel path-maps) instead of the action env dict (which Bazel does not path-map).

Test plan

• Build with and without --experimental_output_paths=strip and verify builds succeed
• Build with --compilation_mode=fastbuild then --compilation_mode=opt and verify increased cache hit rate
• Verify cargo build scripts receive correct OUT_DIR paths